Titanium oxide is a material which is widely known as a photocatalyst, but almost nonfunctional in a place without ultraviolet rays. Therefore, extensive research has been conducted on a tungsten oxide photocatalyst which can use visible light.
A photocatalyst using tungsten oxide particles singly generates holes in a valence band and electrons in a conductive band, respectively, by light excitation under irradiation of visible lights. However, since the conductive band has an energy level lower than a redox potential, the oxygen cannot be reduced with the electrons excited in the conductive band and the generation of active oxygen species is not sufficient. As a result, the photocatalyst does not show photocatalyst activity in an environment under visible light irradiation.
Accordingly, in an attempt to improve the catalyst activity under visible light irradiation, a catalyst having cocatalyst supported on the surface of tungsten oxide has been proposed. For example, a catalyst supporting platinum can show photocatalyst activity under visible light irradiation (JP-A-2009-160566 (U.S. Pat. No. 8,017,238); Patent Document 1). However, noble metals such as platinum have a problem of high costs due to their scarcity. On the other hand, a tungsten oxide photocatalyst having relatively-inexpensive copper supported thereon as copper ion or copper oxide has been proposed (JP-A-2008-149312; Patent Document 2, JP-A-2009-226299 (U.S. Patent Publication No. 2011/005916 A1); Patent Document 3).
An attempt to combine a tungsten oxide catalyst with the other photocatalysts has also been made in order to improve the photocatalyst activity. For example, it has been disclosed that a catalyst combining nitrogen-doped titanium oxide and tungsten oxide and a catalyst combining titanium oxide supporting iron oxide and zeolite supporting tungsten oxide (photocatalyst body) show high photocatalyst activity (JP-A-2007-98294 (U.S. Patent Publication No. 2008/0119352); Patent Document 4). Furthermore, a photocatalyst body has also been proposed which allows titanium oxide to coexist with tungsten oxide and has electron-withdrawing material or a precursor thereof containing at least one metal atom selected from Cu, Pt, Au, Pd, Ag, Fe, Nb, Ru, Ir, Rh and Co supported on at least either of titanium oxide and tungsten oxide (JP-A-2011-20009; Patent Document 5). However, since two kinds of photocatalysts are mixed by simple kneading by a dry or wet method in the photocatalyst bodies of Patent Documents 4 and 5, it is difficult to mix nanoparticles of titanium oxide and tungsten oxide uniformly at nanoscale. As a result, high catalyst activity has not been attained with respect to the catalysts of the documents.
Photocatalysts undergo changes in color in some cases when they are used under light irradiation due to the degradation of the catalyst itself or aggregation of the metal particles of the cocatalyst, and some measures should be taken to prevent it.